Diamond has high hardness, high abrasion resistance, high compressibility and low heat expansion. Although it is an insulator, diamond has very high heat conductivity. With high refractive index diamond is transparent to ultraviolet light, visible light and infrared light. It excels in chemical resistance and in propagation velocity of sound waves. Although diamond is inherently an insulator, it can be converted to a p-type semiconductor or an n-type semiconductor by doping. Thus, applications of diamond have been intensively contrived in various fields of technologies. Diamond is a very important material in present industries.
Bulk diamond can be synthesized by the ultrahigh pressure method. Film diamond is synthesized by a microwave plasma chemical vapor deposition (CVD) method, or a thermal filament CVD method, etc. from a vapor phase consisting of the mixture gas including hydrocarbon gas and hydrogen gas. Film diamond has excellent characteristics similar to bulk diamond. Thus, film diamond has also developing the scope of the applications.
The body on which diamond films are deposited is called a substrate body. A substrate may mean a plate, i.e. two dimensional body in general. In this invention, a substrate shall include a three dimensional body besides a plate as a base body of deposition. The conventional vapor deposition methods required heating the substrate body up to 700.degree. C.-1000.degree. C. Accordingly, the material of the substrate body has been restricted to materials with high heat resistance. For example, diamond, molybdenum, silicon or other refractory materials are used as substrate bodies in the conventional CVD methods of diamond.
However, the vapor phase deposition of diamond films has recently been investigated intensively. Some researchers have reported that diamond can be synthesized at as low as 400.degree. C. Such low temperature synthesis of diamond has made it possible to use a greater array of materials for the substrate body. Although diamond can be produced at low temperature of 300.degree. C., such a low temperature deposition is insufficient to produce diamond films of high quality with high yield. Low temperature synthesis of diamond which can produce diamond of high quality with high yield is strongly desired. In order to lower the temperature of synthesis, adding halogen gas to material gas has been proposed. Halogen gas means chloride gas, fluoride gas, bromide gas and compound gas including halogen.
The low temperature synthesis still has drawbacks. First, the growing speed of film diamond is very slow. Second, the diamond synthesized by the low temperature method, for example at temperatures as low as 400.degree. C. is often accompanied by nonhomogeneous opaque parts. The diamond is inferior in strength or abrasion resistance to the diamond produced at high temperature. Low yield and low quality are fatal drawbacks of the present low temperature method of diamond synthesis. Opacity, weak strength or low abrasion resistance originates from the inclusion of non-diamond ingredients; e.g. amorphous carbon or graphite included in the diamond grown at low temperature.
The reason why so much non-diamond ingredients are generated in the diamond grown at low temperature has not been understood yet. Even if the conditions for synthesis are all the same; e.g. components of gases, temperature of substrate and pressure for deposition, the fluctuation of the quality of the diamond films were so large that the products using the diamond films were of bad quality. The purpose of the invention is to provide a method for producing diamond of high quality which includes little non-diamond ingredients; e.g. amorphous carbon or graphite by clarifying the reason why the non-diamond ingredients are generated in the low temperature CVD.
Japanese patent publication NO. 3-33641 (33641/91) has proposed a method for producing carbon films by ionizing material gas including halogen atoms, hydrogen atoms and carbon atoms. The material gas is introduced into a reactor and ionized by glow discharge, arc discharge, plasma jet etc. A substrate installed in the reactor has been heated at 200.degree. C. to 900.degree. C. The pressure is reduced to several tens of retort (10.sup.-3 Torr) to several tens of Torr. The carbon films have at least as a short range order a tetrahedral structure like diamond crystal. Doping of halogen atoms is important. The inventors told that the role of the halogen is to ionize or radicalize hydrocarbon or halogenized carbon in the reaction under plasma atmosphere. Such a reaction EQU --C--H+X--C.fwdarw.--C--C+HX
will occur at the surface of the carbon films by the action of halogen. This invention has succeeded in synthesizing diamond-like carbon films at pressures below several tens of Torr which had been the lowest pressure for vapor phase deposition of diamond like films. The lower the pressure is, the lower the impurity concentration becomes. Halogen doping enabled the inventors of 33641/91 to grow amorphous carbon films at low pressure e.g. hundreds of mTorr.
Donald E. Patterson et al have proposed a method for growing diamond films using halogen.
DONALD E. PATTERSON, BENJAMIN J. BAY, C. JUDITH CHU, ROBERT H. HAUGE, AND JOHN L. MARGRAVE, "HALOGEN-ASSISTED CHEMICAL VAPOR DEPOSITION OF DIAMOND", SECOND INTERNATIONAL CONFERENCE, WASHINGTON, D.C. USA, SEP. 23-27, 1990, pp433-438.
The conventional CVD method had used a substrate heated up to 700.degree.-900.degree. C. for growing diamond in vapor phase. But Patterson et al have asserted their method can grow diamond films on a substrate heated even at 250.degree. C. The role of halogen is to facilitate the formation of diamond because of large decrease of Gibb's free energy In the reaction from methane and fluoride to carbon (solid) and hydrofluoric acid. Then, halogen deprives the film of graphite by the following reaction EQU C (graphite)+2F.sub.2 .fwdarw.CF.sub.4
Halogen doping to material gas is effective to lower the temperature of the substrate. Therefore, the method using material gas including halogen has been known. Although the material gas including halogen is used for synthesizing diamond at low temperature, the method is not still so capable for reducing concentration of non-diamond ingredients. Many diamond films which have been grown on a substrate body kept below 400.degree. C. contain large amount of non-diamond ingredients. Such non-diamond ingredients deteriorate the quality of diamond films, e.g. opacity, low abrasion resistance, low growing speed and low strength.